Add a new level to less_rectangular_check, which is less aggressive
and can be used at speed 0 and 1.

Tested encoding speed over 15 sequences with QP=40.
Overall speed gains:
4.8% for speed 0; 2.9% for speed 1.

Compression performance(30 frames, ovr_psnr):
             lowres       midres
speed 0:      0.00%        0.00%
speed 1:     -0.01%       -0.02%

STATS_CHANGED

Change-Id: I974111393b8a13baf5f594539f85ec48ab3cd2fb